Improve agent readiness: add AGENTS.md, .gitignore, and markdownlint config

.gitignore

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+node-v*/
+*.tar.xz
+SHASUMS256.txt
+SHASUMS256.txt.asc
+.env
+.DS_Store
+.vscode/
+.idea/
+*.swp
+*.swo
+*~

.markdownlint.yaml

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+# Exclude line length rule to allow longer documentation lines
+MD013: false

AGENTS.md

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+# AGENTS.md - Development Guide for AI Agents
+
+## Quick Start
+
+This repository builds minimal Node.js Docker images. To build and test locally:
+
+```bash
+# Build Node.js binary for a specific version
+./build.sh -n 20.10.0
+
+# Test the generated binary
+docker build -t node-minimal .
+docker run --rm node-minimal -e "console.log('Hello from Node.js ' + process.version)"
+```
+
+## Build Process
+
+The `build.sh` script compiles Node.js from source as a static binary:
+
+- **Usage:** `./build.sh -n NODE_VERSION`
+- **Example:** `./build.sh -n 20.10.0`
+- **Output:** Creates `node-v*/` directory with compiled Node.js binary
+- **Configuration:** Uses `--fully-static --enable-static --without-npm --without-intl` flags
+- **Duration:** Compilation takes 10-30 minutes depending on system and Node.js version
+
+## Dockerfile
+
+The minimal Dockerfile uses `FROM scratch` and copies only the Node.js binary:
+
+```dockerfile
+FROM scratch
+COPY --link node /bin/
+ENTRYPOINT ["/bin/node"]
+```
+
+## CI/CD Workflows
+
+### dockerimage.yml
+
+- Builds Docker images on PR changes to Dockerfile/build.sh
+- Tests on both linux/amd64 and linux/arm64 platforms
+- Uses ccache for faster compilation
+- Automatically detects latest Node.js version
+
+### update-current-image.yml
+
+- Runs daily on schedule (cron: `30 0 * * *`)
+- Checks for new Node.js releases
+- Builds and publishes to Docker Hub and GitHub Container Registry
+- Tags with version, major version, and "current"
+
+### checkshell.yml
+
+- Validates shell script formatting with shfmt
+- Runs shellcheck for shell script linting
+
+## Scripts
+
+### build.sh
+
+Compiles Node.js statically:
+
+- Fetches GPG keys for signature verification
+- Downloads Node.js source tarball and checksums
+- Verifies GPG signature
+- Configures with fully-static compilation flags
+- Patches build files to work around static linking issues
+- Compiles with optimal parallelization
+
+### check-missing-versions.sh
+
+Checks for new Node.js versions to build:
+
+- Queries GitHub Node.js repository
+- Filters out broken builds
+- Returns newest version to build
+
+## Code Quality
+
+- **Linting:** shellcheck (shell script linting)
+- **Formatting:** shfmt (enforces consistent shell style)
+- **Branch Protection:** main branch requires passing checks and code owner review
+
+## Testing
+
+Integration tests run in CI:
+
+- Build Docker image with latest Node.js
+- Execute JavaScript code via docker run
+- Verify Node.js version output
+
+## Dependencies
+
+External dependencies (handled by build.sh):
+
+- curl - downloads Node.js sources and GPG keys
+- gpg - verifies GPG signatures
+- tar - extracts Node.js source
+- gcc/make - compiles Node.js from source
+- Docker - for building and testing Docker image
+
+## Versioning
+
+Images are tagged with:
+
+- Exact version: `20.10.0`
+- Major version: `20`
+- Latest: `latest`
+- Current: `current` (always latest daily build)
+
+Published to:
+
+- Docker Hub: `chorrell/node-minimal:TAG`
+- GitHub Container Registry: `ghcr.io/chorrell/node-minimal:TAG`
+
+## Environment Variables
+
+Required for publishing (GitHub Actions secrets):
+
+- `DOCKERHUB_USERNAME` - Docker Hub username
+- `DOCKERHUB_TOKEN` - Docker Hub authentication token
+- `CR_PAT` - GitHub Container Registry personal access token